Abstract. A statistical method was developed to extract baseline levels of ground level ozone
in Canada and the US, and to quantify the temporal changes of baseline ozone levels
on annual, seasonal, diurnal and decadal scales for the period 1997 to 2006
based on ground-level observations from 97 non-urban monitoring sites.
Baseline ozone is defined here as ozone measured at a given site in the absence of
strong local influences. The quantification of baseline levels involved using a
Principal Component Analyses (PCA) to derive groups of commonly-varying
sites in contiguous regions by season, followed by using backward air parcel
trajectories to systematically select ozone mixing ratios associated with
the baseline condition in each of the PCA-derived regions. Decadal trends were
estimated by season for each of the regions using a generalized linear mixed
model (GLMM).

Baseline ozone mixing ratios determined by this method were found to vary
geographically and seasonally. For the 1997–2006 period, baseline mixing ratios
were calculated for annual and seasonal periods in seven regions of North
America based on multi-site multi-year averages of the baseline data sets. The
annual average (±1 standard deviation) baseline mixing ratios for the regions
are as follows: Continental Eastern Canada=30±9 ppb, Continental
Eastern US=30±10 ppb, Coastal Eastern Canada=27±9 ppb, Coastal
Western Canada=19±10 ppb; Coastal Western US=39±10 ppb,
Continental Western Canada=28±10 ppb and Continental Western
US=46±7 ppb. Trends in the baseline mixing ratios were also found to vary by
season and by geographical region. On a decadal scale, increasing baseline ozone
trends (temperature-adjusted) were observed in all seasons along the Pacific
coasts of Canada and the US, although the trends in California were not
statistically significant. In the coastal zone of Pacific Canada, positive
trends were found with a rate of increase of 0.28±0.26, 0.72±0.55,
and 0.93±0.41 ppb/a in spring (MAM), summer (JJA) and winter (DJF), respectively. In the Atlantic
coastal region, the trends were also positive in 3 of the 4 seasons (but
only significantly so in MAM). In the high ozone precursor emission areas of
the Eastern United States, decadal trends in baseline ozone are, in general,
negative in the spring, summer and fall and appear to be controlled by the
strong within-region changes induced by decreasing ozone precursor
emissions.